Prediction of retention data from stationary phase constants in gas chromatography

Summary Specific retention volumes and retention indices for selected compounds can be predicted from different sets of stationary phase constants by multiple regression. Errors in the corresponding calculated retention times are between 5 and 15%. Intercept (A) and slope (B) values are given for 72 McReynolds stationary phases. The A value can be predicted from the retention index of benzene with a standard deviation of 9%.     

Model for predicting comprehensive two-dimensional gas chromatography retention times

A model for approximating the relative retention of solutes in comprehensive two-dimensional gas chromatography (GCxGC) is presented. The model uses retention data from standard single-column temperature-programmed separations. The one-dimensional retention times are first converted into retention indices and then these indices are combined in a simple manner to generate a retention diagram. A retention diagram is an approximation of the two-dimensional chromatogram that has retention order and spacing in both dimensions similar to that found in the experimental chromatogram. If required, the retention diagram can be scaled to more closely resemble the two-dimensional chromatogram. The model has been tested by using retention time data from single-column gas chromatography-mass spectrometry and valve-based GCxGC. A total of 139 volatile organic compounds (VOCs) were examined. Approximately half of the VOCs had a single functional group and a linear alkyl chain (i.e., compounds with the structure Z-(CH(2))(n)-H). The retention diagrams had primary retention orders that were in excellent agreement with the GCxGC chromatograms. The relative secondary retention order for compounds with similar structures was also accurately predicted by the retention diagram. However, the relative secondary retention for compounds with dissimilar structures, such as acyclic alcohols and multi-substituted alkylbenzenes, were less accurately modeled. This study demonstrates how readily available single-column retention time data can be used to provide an a priori estimate of the relative retention of solutes in a GCxGC chromatogram. Such a capability is useful for screening possible combinations of stationary phases.     

Solvation parameters Part 1. Mutual improvements of several approaches and determination of two first sets of optimized values

An improvement in the characterization and the determination of the solvation parameters allows, not only a better knowledge of solutions, but also of some biological phenomena. In this paper, we test several published data and approaches in the field of solubility and solvation parameters in two ways: (i) the mutual independence of the parameters and (ii) their ability to take into account recently published gas-liquid chromatographic data. From this enquiry it arises that the most suitable published values are those of Abraham concerning 314 solutes. It also arises that the parameters of dispersion and orientation of this published data set are appreciably improved using two simple equations. In addition, a new set of optimized values for 133 solutes is given, by derivation from retention indices in gas-liquid chromatography (GLC) on five selected stationary phases, published by Kováts and co-workers and in the present study. The two sets have a total of 373 defined compounds.     

Comparative evaluation of retention of hydrocarbons present in the C5-petroleum fraction on methylsilicone and squalane phases

Summary The retention of hydrocarbons present in the C₅ pyrolysis fraction of gasolines on the stationary phases squalane and methylsilicone oil JXR at 30, 40 and 50°C was investigated by capillary gas chromatography. The unified retention indices of the hydrocarbons were also calculated on squalane. The retention indices obtained on these two phases were interrelated and the quantitative relationship with the structure of the solutes was studied. Equations based on the unified retention indices calculated on squalane and some selected structural elements of the solutes permit the calculation of their retention on methylsilicone with sufficient accuracy.     

Prediction of the Retention Indices of Benzene and Methylbenzenes Based on their Retention Data-Physico Chemical Properties Relationship

Retention indices of benzene and 12 methylbenzenes were determined on two capillary columns coated with HP-5 and ZB-WAX at 100 °C. Comparison with the retention indices published by other researchers yields a good accordance. The relationship between the retention index values and several physico-chemical properties of these solutes (freezing point, boiling point, molar volume, van der Waals volume, molar refraction, refraction index, connectivity index, dipole moment and vapor pressure) was investigated by multiple linear regression analysis (MLRA). MLRA equations of seven and eight variables predicted retention indices with mean deviations of 0.66–0.97 index-units in squalane and Carbowax 20 M, and 0.76–1.25 index-units in HP-5 and ZB-WAX. PCA was applied to the system of solutes retention data and stationary phases used, giving results consistent with those of MLR analysis. Samples are scattered or joined by curves. An arrangement of the stationary phases according to the polarity is observed. TCEP was one outlier since it is much more polar than any others.Two groups of variables can be envisaged, one of which is close to the origin of the coordinates, the physicochemical properties that depend linearly. Solute vapor pressure and molar volumes were outliers.     

Insights into the retention mechanism on an octadecylsiloxane-bonded silica stationary phase (HyPURITY C18) in reversed-phase liquid chromatography

Plots of the retention factor against mobile phase composition were used to organize a varied group of solutes into three categories according to their retention mechanism on an octadecylsiloxane-bonded silica stationary phase HyPURITY C18 with methanol-water and acetonitrile-water mobile phase compositions containing 10-70% (v/v) organic solvent. The solutes in category 1 could be fit to a general retention model, Eq. (2), and exhibited normal retention behavior for the full composition range. The solutes in category 2 exhibited normal retention behavior at high organic solvent composition with a discontinuity at low organic solvent compositions. The solutes in category 3 exhibited a pronounced step or plateau in the middle region of the retention plots with a retention mechanism similar to category 1 solutes at mobile phase compositions after the discontinuity and a different retention mechanism before the discontinuity. Selecting solutes and appropriate composition ranges from the three categories where a single retention mechanism was operative allowed modeling of the experimental retention factors using the solvation parameter model. These models were then used to predict retention factors for solutes not included in the models. The overwhelming number of residual values [log k (experimental) - log k (model predicted)] were negative and could be explained by contributions from steric repulsion, defined as the inability of the solute to insert itself fully into the stationary phase because of its bulkiness (i.e., volume and/or shape). Steric repulsion is shown to strongly depend on the mobile phase composition and was more significant for mobile phases with a low volume fraction of organic solvent in general and for mobile phases containing methanol rather than acetonitrile. For mobile phases containing less than about 20 % (v/v) organic solvent the mobile phase was unable to completely wet the stationary phase resulting in a significant change in the phase ratio and for acetonitrile (but less so methanol) changes in the solvation environment indicated by a discontinuity in the system maps.     

Application of target factor analysis of gas chromatography. Reproduction,prediction and classification

Summary Using the method of target factor analysis (TFA) described by Malinowski and Howery a computer program has been developed to study different sets of gas chromatographic retention data. Physico-chemical, topological and uniqueness parameters have been found to be basic factors to describe solute behaviour problems. Factor analytical solutions have been used to reproduce the data matrices and to make predictions based on best sets of basic factors. The mean absolute error in the reproduction step is between 1.72 retention index units (i.u.) for a relatively simple matrix consisting of retention indices of alcohols and 7.36 i.u. for a combined data matrix of alcohol, aldehyde and ketone retention indices. TFA has also been used to classify solutes based on their retention behaviour. Alkanes have been classified from cycloalkanes, alkanes from alkenes, and alcohols from aldehydes and ketones using only their retention data and a special kind of uniqueness vector.     

Lattice-fluid model for gas-liquid chromatography

Lattice-fluid models describe molecular ensembles in terms of the number of lattice sites occupied by molecular species (r-mers) and the interactions between neighboring molecules. The lattice-fluid model proposed by Sanchez and Lacombe (Macromolecules, 1978;11:1145-1156) was used to model specific retention volume data for a series of n-alkane solutes with n-alkane, polystyrene, and poly(dimethylsiloxane) stationary liquid phases. Theoretical equations were derived for the specific retention volume and also for the temperature dependence and limiting (high temperature) values for the specific retention volume. The model was used to predict retention volumes within 10% for the n-alkanes phases; 22% for polystyrene; and from 20 to 70% for PDMS using no adjustable parameters. The temperature derivative (enthalpy) could be calculated within 5% for all of the solutes in nine stationary liquid phases. The limiting value for the specific retention volume at high temperature (entropy controlled state) could be calculated within 10% for all of the systems. The limiting data also provided a new chromatographic method to measure the size parameter, r, for any chromatographic solute using characteristic and size parameters for the stationary phase only. The calculated size parameters of the solutes were consistent, i.e. independent of the stationary phase and agreed within experimental error with the size parameters previously reported from saturated vapor pressure, latent heat of vaporization or density data.     

Retention in gas-liquid chromatography with a polyethylene oxide stationary phase: molecular simulation and experiment

Configurational-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble were carried out to investigate the partitioning of normal alkanes, primary and secondary alcohols, symmetric alkyl ethers and arenes between a helium vapor phase and a polyethylene oxide stationary phase (M(W)=382 g mol(-1)). The united-atom version of the transferable potentials for phase equilibria force field was used to model all solutes, polyethylene oxide and helium. The Gibbs free energies of transfer and Kovats retention indices of the solutes were calculated directly from the partition constants at two different temperatures, 353 and 393 K. Chromatographic experiments on a Carbowax 20M retentive phase were performed for the same set of solutes and temperatures ranging from 333 to 413 K. The predicted retention indices for alcohols, ethers and arenes are overestimated by about 120, 70 and 20 retention index units, respectively, pointing to an overestimation of the first-order electrostatic interactions in the model system. Molecular-level analysis shows that hydrogen-bonding and dipole-dipole interactions lead to orientational ordering for the alcohol and ether analytes, whereas the weaker dipole-quadrupole interactions for the arene solutes are not sufficient to induce orientational ordering. The retention indices of alcohols and ethers decrease with increasing temperature because of the large entropic cost of hydrogen-bonding and orientational ordering. In contrast, the retention indices for arenes increase with increasing temperature because the entropic cost of cavity formation is smaller for arenes than for comparable alkanes.     

A unified classification of stationary phases for packed column supercritical fluid chromatography

The use of supercritical fluids as chromatographic mobile phases allows to obtain rapid separations with high efficiency on packed columns, which could favour the replacement of numerous HPLC methods by supercritical fluid chromatography (SFC) ones. Moreover, despite some unexpected chromatographic behaviours, general retention rules are now well understood, and mainly depend on the nature of the stationary phase. The use of polar stationary phases improves the retention of polar compounds, when C18-bonded silica favours the retention of hydrocarbonaceous compounds. In this sense, reversed-phase and normal-phase chromatography can be achieved in SFC, as in HPLC. However, these two domains are clearly separated in HPLC due to the opposite polarity of the mobile phases used for each method. In SFC, the same mobile phase can be used with both polar and non-polar stationary phases. Consequently, the need for a novel classification of stationary phases in SFC appears, allowing a unification of the classical reversed- and normal-phase domains. In this objective, the paper presents the development of a five-dimensional classification based on retention data for 94-111 solutes, using 28 commercially available columns representative of three major types of stationary phases. This classification diagram is based on a linear solvation energy relationship, on the use of solvation vectors and the calculation of similarity factors between the different chromatographic systems. This classification will be of great help in the choice of the well-suited stationary phase, either in regards of a particular separation or to improve the coupling of columns with complementary properties.     

Combined supercritical fluid chromatographic methods for the characterization of octadecylsiloxane-bonded stationary phases

In this paper, we present a combination of a key-solute test based on retention and separation factors of large probe solutes (carotenoid pigments) and a quantitative structure-retention relationship analysis based on the retention factors of small probe solutes (aromatic compounds) to investigate the different chromatographic behavior of octadecylsiloxane-bonded stationary phases of all sorts: classical, protected against silanophilic interactions or not, containing polar groups (endcapping groups or embedded groups). Varied chemometric methods are used to enlighten the differences between the 27 phases tested. The results indicate that the two approaches chosen (carotenoid test and solvation parameter model) are complementary and provide precise information on the chromatographic behavior of ODS phases.     

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model IV. Aromatic stationary phases

The purpose of the present work was to systematically study the chromatographic behaviour of different aromatic stationary phases in a subcritical fluid mobile phase. We attempted to assess the chemical origin of the differences in retention characteristics between the different columns. Various types of aromatic stationary phases, all commercially available, were investigated. The effect of the nature of the aromatic bonding on interactions between solute and stationary phases and between solute and carbon dioxide-methanol mobile phase was studied by the use of a linear solvation energy relationship (LSER): the solvation parameter model. This study was performed to provide a greater knowledge of the properties of these phases in subcritical fluid chromatography, and to allow a more rapid and efficient choice of aromatic stationary phase in regard of the chemical nature of the solutes to be separated. Charge transfer interactions naturally contribute to the retention on all these stationary phases but are completed by various other types of interactions, depending on the nature of the aromatic group. The solvation vectors were used to compare the different phase properties. In particular, the similarities in the chromatographic behaviour of porous graphitic carbon (PGC), polystyrene-divinylbenzene (PS-DVB) and aromatic-bonded silica stationary phases are evidenced.     

The effect of the structure and structural changes of the stationary phase on the retention characteristics of solutes. I. The study of interactions in solutions

Summary Interactions between n-octadecane and its monofunctional derivatives (1-chloro-, 1-hydroxy-), used as stationary phases, and solutes of various structural types have been studied. Using open tubular columns, ensuring a high separation efficiency and relative inertness of the inner surface, the retention data were measured and from them the specific retention volumes, the retention indexes, and the differential solution heats were calculated. Correlations between the measured quantities and the structures of both the stationary phases and the solutes are discussed.     

Characterization of stationary/mobile phase combinations by using markers : Part 1. Preliminary results for the prediction of reversed-phase retention on various stationary phases

The characterization of stationary/mobile phase combinations can be done in a phenomenological way by measuring the k′ values of specific solutes, the markers. These markers can be chosen optimally from a set of test solutes with the use of multivariate techniques. When retention data of solutes on different stationary phases, with varying mobile-phase compositions, are available, a procedure is given to predict the retention of those solutes on new stationary phases. This procedure uses markers to characterize the new stationary/mobile phase combinations.